1. Field of the Invention
The present invention relates to switching apparatus for use, for example, in asynchronous transfer mode (ATM) networks.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows parts of switching apparatus previously considered for use in an ATM network. The switching apparatus 1 includes a plurality of data units 2.sub.0 to 2.sub.3, each data unit having an input portion 4.sub.0 to 4.sub.3 and an output portion 6.sub.0 to 6.sub.3. In FIG. 1, the output portion 6 of each data unit 2 is shown separately from the input portion 4 of the unit concerned, so as to clarify the operation of the apparatus, but it will be appreciated that both the input and output portions 4 and 6 form part of the same data unit 2.
Each data unit 2 is connected to one or more data input/output ports DP.sub.0 to DP.sub.7 of the switching apparatus; for example, as shown in FIG. 1, each data unit is connected to a pair of data ports DP.
The apparatus 1 further comprises a cross-connect switching unit 8 having as many input ports IP.sub.0 to IP.sub.3 and as many output ports OP.sub.0 to OP.sub.3 as there are data units 2. Each input portion 4 is connected to one of the input ports IP and each of the output portions 6 is connected to one of the output ports OP.
The cross-connect switching unit 8 is controllable selectively to provide connections between its input ports and its output ports. Up to four simultaneous connections, each between one of its input ports and one of its output ports, can be provided. For example, if data is received by the apparatus 1 at data port DP.sub.2 (the "source" data port) that is to be routed to data port DP.sub.4 (the "destination" data port), the cross-connect switching unit 8 is configured to provide a connection between the input portion 4.sub.1 of the data unit 2.sub.1 (the "source data unit" having the source data port DP.sub.2 connected to it) to the output portion 6.sub.2 of the data unit 2.sub.2 (the "destination data unit") which has the destination data port DP.sub.4 connected to it. Thus, the source data unit 2.sub.1 and the destination data unit 2.sub.2 form a data-unit pair and are allocated a data transfer path within the apparatus for use in passing data from the source data unit of the pair to the destination data unit of the pair. At the same time, up to three other such data-unit pairs can be allocated respective data transfer paths by the switching unit 8 to enable data received at source data ports connected with the respective source data units of those pairs also to be routed through the switch to the respective destination data units of the pairs, those destination data units being connected with the relevant destination data ports.
Since it is possible for two (or more) source data ports to wish to communicate at the same time to the same destination data port, it is normal to make provision for some buffering of the data at some point within the switching apparatus, either within the data units 2 or in the cross-connect switching unit 8, or possibly in both. This buffering holds up the data of one of the two contending source data ports pending transfer to the intended destination data port of the data received at the other of those two contending source data ports.
In practice, switching apparatus for use in an ATM network may require a large number of data ports and hence a large number of data units 2. For example, it may be desired to provide ATM switching apparatus with several hundred data ports. This inevitably requires a large number of data units 2 and a correspondingly-large number of input ports and output ports on the cross-connect switching unit 8.
However, the number of input ports and output ports which a switching unit can have is limited when (as is generally desired) the switching unit is formed as an integrated circuit. For example, it is not readily practicable to provide an integrated circuit package with more than 300 connection pins at the sorts of data transfer rate required in ATM switching applications.
It is accordingly desirable to provide switching apparatus in which the above-mentioned restriction on the number of input and output ports of the cross-connect switching unit no longer limits the size of the switching apparatus.